Stochastic scanning documents to change moire effects

ABSTRACT

An additional motion is introduced into a scanning operation. This additional motion is in addition to the motion that is inherent in any document scanning operation. For example, in a flatbed scanner a linear array of scanning devices is positioned across the document in a first or “x” direction and this array is moved across the document in a perpendicular or “y” direction. In a flatbed scanner the position of the scanning devices are fixed in the “x” direction. In a drum scanner a light beam moves across the document in an “x” direction and the document is moved in the “y” direction. The light reflected from the document is directed to a fixed photoreceptor. With the present invention an additional vibratory motion is introduced into the scanning process. With a flatbed scanner the array of detector devices is moved or vibrated. The vibratory motion can for example be in the “x” direction. With a drum scanner the photoreceptor is moved or vibrated. For example the photodetector can be moved or vibrated in the “x” direction. The amount and direction of the vibratory movement can vary up to about one half the distance between units in the scanner&#39;s resolution (i.e. one half a pixel). If one is trying to eliminate Moiré patterns the vibratory movement can be a pseudo random series of movements. If one is trying to introduce a special pattern into the document or to otherwise create special effects, various other type of motion can be used. Movement of the CCD array in a flatbed scanner or movement of the photoreceptor in a drum scanner can be done by a simple piezo electric transducer or by a simple mechanical cam.

FIELD OF THE INVENTION

The present invention relates to document scanning and more particularlyto a method and apparatus for changing the Moiré effects introduced byconventional scanning apparatus.

BACKGROUND OF THE INVENTION

Existing document scanners can generally be classified as either flatbedscanners or drum scanners. Flatbed scanners generally have a lineararray of detecting devices such as linear charge coupled devices (CCDs).The space between the detecting devices determines the horizontalresolution of the scanning device. The speed that the array is movedalong the document and the rate at which the devices are read,determines the vertical resolution of the scanning device. Flatbedscanning devices which have 300 to 600 CCD elements per inch arecommercially available.

Drum scanners generally have a single photo-detector and a scanning beamof light. The beam of light scans across a line of the document as thedocument moves around the drum. The light reflected from the document isdirected to the single photodetector. The horizontal resolution of thescanner is primarily determined by the frequency that the output of thesingle photoreceptor is sampled as the light beam moves across a line ofthe document.

Drum scanners can also include a linear array of detectors and there aremany different types of scanners which combine elements from the abovedescribed two general types of scanners.

Scanning a document at a particular resolution (for example at 600pixels per inch) can introduce patterns into the data. The patternsintroduced into data by scanning appear when the scanned data is printedor displayed. Such patterns are frequently referred to as Moirépatterns.

In some instances special patterns are intentionally introduced into animage so that when the image is scanned and then reproduced the Moirépatterns produce visible images. When such images are visible itindicates that a document is not an original, that is, the image wascreated by scanning an original. Such technology is well developed andis for example described in issued U.S. Pat. No. 5,735,547.

The present invention is directed to a method and apparatus for scanninga document in a manner which changes how and whether Moiré patterns areintroduced into an image when the image is scanned.

SUMMARY OF THE INVENTION

With the present invention an additional motion is introduced into thescanning operation. This additional motion introduced by the presentinvention is in addition to the motion that is inherent in any documentscanning operation. For example, in a flatbed scanner a linear array ofscanning devices is positioned across the document in a first or “x”direction and this array is moved across the document in a perpendicularor “y” direction. In a flatbed scanner the position of the scanningdevices are fixed in the “x” direction. In a drum scanner a light beammoves across the document in an “x” direction and the document is movedin the “y” direction. The light reflected from the document is directedto a fixed photoreceptor. With the present invention an additionalvibratory motion is introduced into the scanning process. With a flatbedscanner the array of detector devices is moved or vibrated. Thevibratory motion can for example be in the “x” direction. With a drumscanner the photoreceptor is moved or vibrated. For example thephotodetector can be moved or vibrated in the “x” direction. The amountand direction of the vibratory movement can vary up to about one halfthe distance between units in the scanner's resolution (i.e. one half apixel). If one is trying to eliminate Moiré patterns the vibratorymovement can be a pseudo random series of movements. If one is trying tointroduce a special pattern into the document or to otherwise createspecial effects, various other type of motion can be used. Movement ofthe CCD array in a flatbed scanner or movement of the photoreceptor in adrum scanner can be done by a simple piezo electric transducer or by asimple mechanical cam.

With the present invention, the data generated by the vibrated scanningdevice can be manipulated to generate a digital image similar to thatwhich would be generated by a normal scanning element, except that thedigital image so generated does not contain the majority of the Moirépatterns that would have been generated by a normal scanning element.The data from the vibrated scanning element can also be manipulated toenhance certain desired effects. That is, the present invention can beused to enhance patterns that are intentionally placed in an image. Forexample, an image can be constructed so that it will contain a patternthat will only be visible if the image is scanned with a scanner whichis operating in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall diagram of a scanning system which utilizes thepresent invention.

FIGS. 2A and 2B are diagrams showing how the CCD scanning array and thepiezo electric transducer are mounted on the frame.

FIG. 3 shows the scanning array in a series of different positions.

FIG. 3A illustrates the pattern of pixels created by a scanner whichdoes not utilize the present invention.

FIG. 4 shows the location on a document of the pixels from the scanningelement.

FIG. 5 shows how the data from the scanner is combined.

FIG. 6 shows a preferred embodiment of the invention.

DETAILED DESCRIPTION

An overall diagram illustrating the present invention applied to aflatbed scanner is shown in FIG. 1. The scanner shown in FIG. 1 has ascanner bed 10 on which is positioned a document 11. A moveable scanningcarriage 12 contains a light source 13 and a linear array of chargecoupled devices (CCDs) 14. A line on the document 11 is illuminated byLED light source 13. An image of the illuminated line is focused on CCDdetector array 14 by a lens (not explicitly shown). The array 14 ismoved across the document in the y direction (by a conventionalmechanical mechanism which is not explicitly shown in the drawing). Amicroprocessor 20 controls the operation of the entire system andsignals from the CCD array 14 are stored in memory 20A. As the systemscans a document 11 the signals from array 14 creates a digital image ofthe document in micro-processor memory 20A. All of the above elementsare conventional and they are found in many commercially availableflatbed scanners. The present invention involves transducer 15 which ispositioned at the end of array 14 and which will be described later withreference to FIGS. 2A and 2B.

FIG. 3A illustrates the image of document 10 which would be created inmemory 20A if the system operated in a conventional manner without thepresent invention. The image has pixels 10 a, 10 b, 10 c, etc. arrangedin uniform rows and columns. The memory 20A stores a number for eachpixel which indicates the intensity of that pixel. For completeness itis noted that a color scanner would store three numbers for each pixelto indicate the intensity of each of the tree primary colors in eachpixel. The preferred embodiment of the invention will be described as asingle color (i.e. a black and white) scanner. However, the inventioncan be extended to a color scanner by handling each color in the samemanner that a single color is handled in the embodiment describedherein.

The speed at which the array 14 is moved in the “y” direction and therate at which the CCD devices 14 are read must be coordinated. Forexample in a scanner which is designed to create an image with 600pixels per inch, if the time required for the array 14 to move 1/600thof an inch is one millisecond, a reading must be taken from the CCDelements in array 14 each millisecond. That is, the rate at which datais read from CCD devices in array 14 must be coordinated with the speedthat the array 14 is moved in the “y” direction so that a new row ofpixels is in position to be read each time the CCD elements in array 14are read. Scanners which produces the type of image illustrated in FIG.3A are commercially available. The difference between the presentinvention and the prior art relates to the transducer unit 15 whichmoves or vibrates the array 14 in the “x” direction. As shown in FIG. 2the CCD scanning array 14 has a large number individual CCD devices 14a, 14 b, 14 c etc. One end of the scanning array 14 is attached totransducer unit 15 which may be a piezo electric crystal . As shown inFIG. 2B, the piezo electric transducer 15 is attached to stationaryframe 18. The array of CCD devices is free to move in the x direction(i.e. directions 28 and 29) by sliding in a channel or grove in frame18. When a voltage is applied to piezo electric transducer 15 byelectronic circuitry 19, the piezo electric transducer 15 expands andthe CCD scanning array 14 moves in the direction 28. When the voltage isremoved from the piezo electric transducer, the array 14 moves indirection 29 back to its original position.

In the first embodiment of the invention described herein the scanningarray 14 is moved in the x direction (i.e. in directions 28 and 29) byan amount equal to one half a pixel size each time a row of pixels isread. The sequence of steps is:

a) The first row of pixels is read,

b) The scanning element is moved to the right (direction 28) by one halfa pixel,

c) The second row of pixels is read,

d) The scanning element is moved to the left (direction 29) by one halfa pixel, and

c) The second row of pixels is read.

The sequence is then repeated. FIG. 3 shows the position of CCD scanningarray 14 at three different times T1, T2, and T3. It is noted that inFIG. 3, the CCD devices 14 a, 14 b, 14 c, etc. are shown in an expandedview and thus all of the elements in array 14 can not be shown on thefigure. It should also be noted that the movement of the array 14 in the“y” direction along the document is in fact continuous and the positionsdiscussed here are the positions of the array at the specific times thatthe CCD devices are sampled or read. At time T1, the scanning array 14is in what will be called the “home” position. After the value is readfrom the CCD devices in the home position, the array 14 moves one pixelposition in the “y” direction and the array 14 is moved one half a pixelposition in the “x” direction 28. A second set of values is then readfrom the CCD devices 25 Next the array moves one more pixel position inthe “y” direction and the array 14 is moved one half a pixel positionback to the home position. The process is then repeated.

FIG. 4 shows the document 11 with an indication of the location of thepixels that are read by CCD elements 14 a, 14 b, 14 c, etc. It is notedthat the edge portions of the document are ignored and only theinformation in the area designated by line 41 is of interest. It is alsonoted that since the document is shown enlarged in FIG. 4, only the topleft corner of the document is shown.

The pixels in each row of the document are designated as by two numbers.The first number indicates the row where the pixel is located and thesecond number indicates the position of the pixel in the row. Thus thethird pixel in the second row is designated as “2,3”.

FIG. 5 illustrates the data in memory 17 which represents the data fromarea 41 of document 11. FIG. 5 represents an array of numbers stored ina memory in a conventional manner to represent an image. The images arearranged in rows and columns. Two letters are used to represent eachnumber. For example “B,C” represents the third number in the second row.The number “B,C” is a number stored in memory 20A which represents thevalue of a particular pixel in the image.

The values represented in FIG. 5 are calculated from the values readfrom the CCD devices as they scan each of the pixels shown in FIG. 4.The values in the first row are the direct values read from the scanningelements. For example value “A,B” is the value read from a CCD elementas it scans pixel “1,3” shown in FIG. 4. The values in row B arecalculated values. For example value “B,A” is derived from the valuesread from the CCD elements as they scan pixels “2,1” and “2,2”. Thevalue of “B,A” is one half the value of “2,1” plus one half the value of“2,2”. The reason for this is that the value “B,A” represents a pseudopixel that is located half way between pixels “2,1” and “2,2”. Each ofthe other values for the values in rows 2, 4, 6, etc. are calculated inlike fashion.

From a physical point of view, a first set of values is read when thescanning array 14 is in position shown in T1 in FIG. 3. Next the array14 is moved in the “y” direction by an amount equal to one pixelposition and the scanning array 14 is moved one half a pixel position inthe “x” direction 28 and a second set of values are read from thescanning array 14. The process then continues as previous explained. Ifthe values read as described above were directly printed, on aconventional printer, the image would be distorted, since a printerprints the value of the pixels in the second row of an image directlybeneath and aligned with the pixels in the first row of an image.

The above distortion which would occur due to the fact that the scanningarray is moved between rows of pixels in an image is what necessitatesthe calculation of “pseudo” pixels as previously described.

In the simplified embodiment of the invention described above, thecalculation of he value of the pseudo pixels is relatively easy sincethe scanning element is moved a uniform one half pixel amount betweeneach scan row. In the preferred embodiment of the invention shown inFIG. 6 and described below, the calculation becomes somewhat morecomplicated.

In the preferred embodiment of the invention shown in FIG. 6, thescanning element 25 is moved in accordance with a pseudo random pattern.As shown in FIG. 6, the pseudo random pattern has fifteen differentvalues between 10 and 90 percent. Each value represents the percent ofthe width of a pixel which the scanning element is moved during aparticular time period. For example the scanner might be moved by thefollowing amounts over fifteen rows of pixels:

T1 .27% of a pixel T2 .12% of a pixel T3 .60% of a pixel T4 .19% of apixel T5 .15% of a pixel T6 .19% of a pixel T7 .22% of a pixel T8 .59%of a pixel T9 .00% of a pixel T10 .62% of a pixel T11 .25% of a pixelT12 .02% of a pixel T13 .39% of a pixel T14 .75% of a pixel T15 .78% ofa pixel

The above series of numbers is merely intended as an example. Themathematics for generating a series of pseudo random numbers is welldeveloped. While in general the above numbers will provide satisfactoryresults, in various applications other series with more or less numbersand series generated with other random number generators may be better.The particular numbers used is a matter of choice for each particularapplication.

A system which utilizes a series of random numbers to determine theamount of movement between rows of pixels is shown in FIG. 6. The CCDscanner 25 and the driver 26 are similar to the previous embodiment. Theseries of numbers which specify the amount of movement are stored inmemory 63. A driver control 27 reads the numbers from memory 63 andsequentially activates the driver 26 with signals which have a magnitudecontrolled by the numbers in storage 63. The output of the scanner goesto memory 61. The calculating device 65 combines the values from memory61 into a final image based upon the amount of movement of each line.

In the previous example where the scanning element was moved one half ofa pixel, the numbers in the final image was calculated by taking onehalf of the value of two pixels. In the situation described in thepresent embodiment the calculation is somewhat more complicated. Forexample if the movement is 0.25 of a pixel, the final value of a pixelwould be calculated by taking 0.25% of the value of one value and .75%of another value.

The diagram shown in FIG. 6 shows a number of discrete components. itshould be recognized that the components shown in FIG. 6, could bediscrete components; however, they could also be subroutines and area ofmemory in a single conventional computer.

The present invention can also be used to enhance patterns which areintentionally place in an image. That is, the present invention can beutilized to detect special patterns which are placed in an image andwhich can only be detected by a scanner which operates according to thepresent invention

As a simplified example consider an image which is printed at 1000pixels per inch and which is scanned at 750 pixels per inch. Considerthat the pixels in the image are printed in the pattern given below. Inthe following lines the numbers “9” and “6” indicate the intensity of apixel. The underlining is only added to emphasize the pattern of thepixels.

699669966996699669966996699669966996699669966996699669966996699669966996699669966996699669966996699669966996699669966996699669966996

With a conventional scanner, each scanned pixels would cover three ofthe original pixels and have one of the following values (where thenumbers “24” and “21” indicate the values of the scanned pixels. Theunderlining shows the pixels in the original that are grouped togetherin the scanned pixels.:

69966996699669966996699669966996699669966996  24 21 21 24 24 21 2169966996699669966996699669966996699669966996   24  24 21 21 24 2469966996699669966996699669966996699669966996   21 24  24 21 21

The above shows each scanned lines. With a conventional scanner thelines which appear beneath each other would be as follows for each ofthe above examples:

24 21 21 24 24 21 21 24 21 21 24 24 21 21 24 21 21 24 24 21 21 24 21 2124 24 21 21 or 24 24 21 21 24 24 24 24 21 21 24 24 24 24 21 21 24 24 2424 21 21 24 24 or 21 24 24 21 21 21 24 24 21 21 21 24 24 21 21 21 24 2421 21

With the present invention, where the scanner is shifted two pixels theright after the first row and then shifted back left after the secondrow, the patterns would be as follows:

24 21 21 24 24 21 21 24 24 21 21 24 24 21 21 24 21 21 24 24 21 21 24 2421 21 24 24 21 21 or 24 24 21 21 24 24 21 24 24 21 21 24 24 24 24 21 2124 24 21 24 24 21 21 24 24 or 21 24 24 21 21 21 21 24 24 21 21 21 24 2421 21 21 21 24 24 21 21

As can be seen from the above only every third column has identicalvalues in a vertical row, where with a conventional scanner, eachvertical row was identical. When a printed image of the above is viewed,it would have a different appearance to the viewer.

It is noted that the above is a very simple example to illustrate theprinciple involved. The same principle could be used with a much morecomplicated pattern. Likewise the above illustration is given in termsof the first simplified embodiment of the invention.

The same technique could be used with the preferred embodiment of theinvention where the scanner is shifted in a more complicated pattern.

The above described technique can be used to facilitate thedetermination if documents are originals or copies. Using the abovetechnique a pattern can be printed in a document which can only bedetected by a scanner which has an array of elements that moves in aparticular patterns. If such a document is scanned on a conventionalscanner and then reproduced, the intensity of the hidden image will bereduced or destroyed.

The invention has been described with respect to single color black andwhite scanner. The invention could be similarly applied to a colorscanner. A color scanner generally has three rows of detectors, one foreach color. These detectors could all be moved together in the “x”direction or more complicated patterns could be created by moving thedetectors for each pattern in accordance with a different pattern ofmovement.

It is noted that the present invention may not entirely eliminate Moirépatterns in all instances. However on a stochastic basis the presentinvention will minimize Moire patterns in most conventional images.

In the embodiment described herein, the detector elements are moved inthe “x” direction while the document is canned in the “y” direction. Incertain applications it may be desirable to move the detector elementsin a direction other than in the “x” direction.

The mechanism for moving array 14 in a ““y” direction can beconventional. For example the array 14 could be mounted on rollers andspring biased to move in one “y”” direction. An electric motor with asmall cable could pull the array 14 against the spring bias in order tomove the array over an document during the scanning operation.

While the invention has been shown and described with respect topreferred embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail may be make withoutdeparting from the spirit and scope of the invention.

I claim:
 1. In a scanner which scans documents by moving a linear arrayof detectors over a document in a first direction, at least some of thedetectors in said array each covering a particular distance, theimprovement which comprises vibrating said linear array of detectors ina direction perpendicular to said first direction with a pre-determinedpseudo random pattern of motion, detecting signals from said detectorsto generate an image with pixels each having a width equal to saidparticular distance, whereby Moiré patterns are reduced.
 2. The devicerecited in claim 1 wherein said vibration has an amplitude less than onehalf the width of one of said detectors.
 3. A scanner which includes alinear array of detector elements each of which generates outputsignals, at least some of said detector elements each having a widthequal to a first value, means for moving said array over said documentin a pre-determined pseudo random pattern of motion in a firstdirection, means for moving said array of detectors in a directionperpendicular to said first direction, means for combining the outputsignals from said detectors to generate a square array of pixel valueswhich represent an image, each pixel having a size equal to said firstvalue, whereby Moiré patterns in the said image are reduced.
 4. Ascanner which includes a linear array of detector elements each of whichgenerates output signals, the distance between center points of saiddetector elements being equal to a first value, a device for moving saidarray over said document in a direction “y”, a transducer for movingsaid array of detectors in a direction “x”, direction x beingperpendicular direction “y”, a computational device for combining theoutput signals from said detectors to generate a square array of pixelvalues which represent an image, said pixels having a pixel pitch equalto said first value, whereby Moiré patterns in the said image arereduced.
 5. In a scanner adapted to scan documents by moving a lineararray of charge coupled devices (CCDs) over a document in a firstdirection, the improvement which comprises vibrating in accordance witha pre-determined pseudo random pattern said linear array of CCDs in adirection perpendicular to said first direction in order to reduce Moirépatterns.
 6. The method of scanning a document which consists of aseries of rows of areas with a linear array of detectors, the distancebetween the center points of said detectors equaling a first value, saidrows being arranged in a direction “y”, said method comprising,positioning said detectors over a first row of said document, readingthe value of the output of said detectors to generate output signals,moving said detectors over a second row of said document and also movingsaid detectors in a direction “x”, direction “x” being perpendicular tosaid direction “y”, repeating said positioning, reading and moving stepsuntil said entire document has been scanned a computational device forcombining the output signals from said detectors to generate an imagewith pixels, the pixel pitch of which equal said first value.
 7. Themethod recited in claim 6 wherein said movement in said directionperpendicular to said first direction is done in pseudo random amounts.8. In a scanner which scans documents by moving a linear array ofdetectors over a document in a first direction, each of said detectorscovering a first distance, the improvement which comprises means forvibrating said linear array of detectors in a direction perpendicular tosaid first direction, said vibration being in accordance with a pseudorandom pattern of motion, and generating an image from said detectorswhich has pixels values equal to said first distance, whereby Moirépatterns are reduced.
 9. The device recited in claim 8 wherein saidvibration follows a pseudo random pattern of motion.
 10. The devicerecited in claim 8 wherein said vibration has an amplitude less than onehalf the width of a scanning element.
 11. A flatbed scanner fordocuments, said scanner including a linear array of detectors which ismoved over the length of said document, the length of said linear arrayat least equaling the width of said document, each of said detectorscovering a first distance, means for vibrating said linear array in adirection perpendicular to the length of said document, said vibrationbeing in accordance with a pseudo random pattern of motion, means forgenerating an image which has a pixel size equal to said first distancefrom the output of said detectors, whereby Moiré patterns are reduced.12. The scanner recited in claim 11 wherein said means for vibratingincludes a piezo electric crystal.
 13. The scanner recited in claim 11wherein said vibration is according to a pseudo random pattern.
 14. Thedevice recited in claim 11 wherein said vibrator movement has anamplitude less than one half the width of a scanning element.
 15. Amethod of scanning a document including moving a linear array ofdetectors over the length of said document, the length of said lineararray at least equaling the width of said document, said detectors eachhaving a width “x”, vibrating said linear array in a directionperpendicular to the length of said document, said vibration being inaccordance with a random pattern of motion, generating an image with apixel pitch “x” from the output of said detectors, whereby Moirépatterns are reduced.
 16. The method recited in claim 15 wherein themagnitude of said vibration is less than one half the width of one ofsaid detectors.